Decline of Growth and the Eest Period. Ill 



ber, to push vigorously in the first warm days of spring. 

 The rest period is to be regarded as a normal, if not a 

 necessary factor of plant life. 



173. Most Plants Under Glass Require Rest from time to 

 time, or they do not thrive. The rest is provided either 

 by keeping them at a lower temperature than is favorable 

 to growth, or by submitting them to a degree of dry ness 

 that prevents growth. The latter is preferable for plants 

 native in the tropics, where they naturally lie dormant 

 during the dry season. 



174. The Time of Leaf Fall is an Index of Wood Maturity 

 in healthy deciduous trees and shrubs. In these, the 

 coloring and fall of the leaves in autumn is not necessarily 

 clue to frost, but results from the dormant condition that 

 accompanies maturity. As a rule, the more mature 

 leaves are precipitated by the first autumn frosts. Those 

 less mature usually remain until the more severe frosts. 

 In trees with well -ripened wood, the leaves at the tip of 

 the shoots usually fall before, or not later than, those on 

 the older parts of the tree. With poorly- matured wood 

 the reverse is the case. In a few deciduous trees, as the 

 beech and some oaks, many of the mature leaves remain 

 on through the winter. 



175. Hardiness Depends upon the Degree to which tlie 

 Dormant State is Assumed. Since the most severe cli- 

 matic extremes coine during the natural rest period of 

 plants, the ability of the plant to endure these extremes 

 depends upon the extent to which the protoplasm be- 

 comes dormant during the decline of growth. As a rule, 

 a given plant is hardy (10) in a locality in which the 

 duration and the warmth of the growing season are suf- 



